1. Field of the Invention
The present invention relates to an image processing apparatus including a density correction unit.
2. Description of the Related Art
As an image recording method used for an image processing apparatus such as a printer or a copy machine, an electro-photographic method is known. The electro-photographic method forms a latent image on a photosensitive drum using a laser beam, and then develops the latent image with charged recording agent (herebelow, referred to as “toner”). A toner image formed with the developed toner is transferred and fixed onto a transfer sheet to record an image thereon.
In this image processing apparatus, after warming up for activation, a specific patch pattern such as a gradation pattern is printed on a recording material such as a sheet. A method is known in which an image reading device such as a scanner reads the above-described patch pattern and gives read information to an image forming condition as feedback so that stability of image quality can be improved. Further, by using the image processing apparatus for a long period of time, an adhesion characteristic of developing tonner to a potential of a photosensitive drum is changed and, as a result, an optimum image forming condition may not be ensured. Therefore, correction is conventionally performed using a relationship between potential data and density.
Furthermore, as the toner image, a plurality of patch patterns on which halftone processing is performed in the apparatus at a predetermined density level are formed on an intermediate transfer member, and then a sensor included in the apparatus measures the density of the patch patterns. Based on the measurement result, a density characteristic of the halftone processing with respect to an input density level is calculated, and then a density correction table is generated so that the input density level of print data has a predetermined standard density value. Subsequently, the input density level of the print data is corrected according to the density correction table. With this correction, colors and the density of an output image are always maintained within a certain region according to an input density level.
In the image processing apparatus of the electro-photographic method, particularly in a quick printing industry, the density correction unit as described above is demanded to further improve density stability. Thus, the number of patch patterns corresponding to the gradation and image formation has being increased to improve the density stability. That is because, for example, the patch patterns are generated that correspond to the increasing number of gradations from a low density level to a high density level, and formation of each image of a low line screen, a high line screen, and error diffusion.
On the other hand, a head-to-sheet distance (between transfer sheet and transfer sheet in continuous pages) is narrowed to improve productivity of the image processing apparatus, so that a non-image forming region on the intermediate transfer member is decreased. Due to the causes described above, the number of patch patterns that can be generated in the non-image forming region is decreased.
Therefore, to ensure the non-image forming region necessary for the number of the patch patterns, the narrowed head-to-sheet distance is widened, or the patch patterns are not collectively generated in one location but generated over a plurality of pages to perform the density correction.
However, extending the time may lower productivity, in other words, extending a time taken from ending the processing on a certain transfer sheet to starting the processing on a following transfer sheet decrease the productivity. Further, generating patch pattern over the plurality of pages, which is the latter case, may deteriorate accuracy of the density stability, since feeding back and reflecting a correction result takes time.
To address such a problem, a technique discussed in Japanese Patent Application Laid-Open No. 2009-237036 controls an exposure unit and a development unit so that a solid image for correction is formed in the non-image region passing an exposure position when a plane phase of a rotating polygonal mirror is corrected. According to the technique, the productivity is prevented from being lowered while a correction operation for stabilizing image quality is performed.
However, according to Japanese Patent Application Laid-Open No. 2009-237036, if the rotating polygonal mirror is not corrected, the problem in which performance is lowered due to the density correction is not fully solved.
Although a system for controlling a timing in which a controller of the conventional image processing apparatus generates the patch pattern can estimate generation of the time sufficient for generating the patch pattern during performing a job, the patch pattern is not generated by efficiently using the time.
The time herein specifically refers to a time for processing of covers, inserting sheets, staples in a procedure of book binding processing, and a time for performing rendering processing in a processing of controller side.
Further, the time is generated due to time related to drum cleaning processing and fixing-roller-temperature adjusting processing performed by an image forming unit. Since the image processing unit and the image forming unit perform in a nonsynchronous manner with each other, the controller may not be able to previously estimate the processing by the image forming unit. Even in such a case, the controller does not calculate the time generated due to the processing by the image forming unit. Therefore, the time is not efficiently used.
Furthermore, even when the time is generated due to the processing performed by either of the image processing unit or the image forming unit, although the patch pattern could be generated and the density correction processing could be performed using the time, the time is not efficiently used.